But his outburst against the police on the Downing Street gates is a serious offence against the second, the iron law that a chief whip should be neither seen nor heard in public.

Braying at the police may go down well among those fellow cyclists who think the rules do not apply to them, but it is at odds with the requirement for whips to put leader and party first at all times.

A chief whip must never allow himself the luxury of a conscience, argued Ted Heath, who did the job for Harold Macmillan, and the same goes for a public profile.

That's why the legendary Westminster whips always deployed their dark arts well away from prying eyes, as detailed in James Graham's new play This House, which opens next week at the National Theatre.

Mr Graham's play deals with the dramatic years of the 1974-9 Labour government, when Labour's whipping operation, masterminded by the fabled Walter Harrison, involved life or death decisions to fend off Margaret Thatcher's Tories.

Hung parliaments demand utterly professional chief whips. Mr Mitchell needs to realise that his first offence this week was to abuse the police. His second was to draw attention to himself.